In a passive state, the pelted mammal is well insulated against loss of thermal energy. Further, the rapidity with which the circulatory system equilibrates thermal energy would render the living animal a useful whole-body calorimeter but for the complicating factors of physiological and behavioral thermoregulation. Moderate thermal loading of the intact mammal by, say, microwave radiation, will bring a host of responses into play by which an excess of energy is actively dissipated. If, however, a reversible chemical lesion could be introduced that rendered the mammal ectothermic–cold blooded–then one could use pre- and post-radiation temperatures of the living animal to make fairly accurate estimates of quantities of absorbed energy. An ectothermic preparation would be useful in dosimetric studies in the free field where an empirical fit between measures of incident and absorbed energy is the objective with the freely behaving animal. We report studies of two compounds, sodium salicylate and cortisone acetate, and their effects upon body temperatures of rats, guinea pigs, and rabbits of both sexes and of pigmented and albino strains. While the salicylate produced a reliable hypothermal response in the albino rat, it was less effective in the pigmented rat, and was virtually ineffective in the guinea pig and rabbit. Cortisone appeared to produce a more uniform hypothermal response; studies of rats revealed that increments of colonic temperature after short periods of highly thermalizing microwave radiation yield estimates of energy dosing that are accurate within 15%.